1. Field of the Invention
This invention relates to balanced output circuit for outputting balanced inverted and non-inverted outputs, and to an electronic apparatus, such as a portable telephone, utilizing such balanced output circuit.
2. Description of the Related Art
In a baseband signal transmission output part and a BTL (Balanced Transformer-less) speaker driver of a portable telephone for example, a balanced output circuit is used which is adapted to provide a non-inverted output that is in phase with an input signal and an inverted output that is out of phase by 180 degrees with the input signal.
In a conventional electric power amplification circuit for use with audio equipment, an audio signal transmitted from a prestage circuit via a capacitor and an inverted feedback signal are inputted to a BTL amplifier. The BTL amplifier provides at the two BTL output terminals thereof an amplified audio signal and an amplified differential signal that is obtained by adding a reference voltage to the difference between the audio signal and the inverted feedback signal. The two output signals are provided to a speaker. The voltage difference between the two amplified signals outputted from the respective BTL output terminals is detected by a differential amplification circuit. The DC component (DC voltage) of the amplified differential signal is extracted by a mirror integration circuit. An inverted feedback signal that is inversely proportional to the DC voltage offset from the reference voltage is fed back to the BTL amplifier. This scheme enables suppression of the DC offset voltage generated across the BTL output terminals of a BTL amplifier. (See, for example, Japanese Patent Application Laid Open, No. H10-93365, which will be referred to as Patent Document 1.)
Such a prior art output circuit as mentioned above requires not only a coupling capacitor for inputting an audio signal received from its prestage circuit, but also a BTL amplifier for providing an inverted and a non-inverted outputs. The output circuit further requires a differential amplification circuit and an error integration circuit for suppressing the DC offset voltage generated in the BTL amplifier. As a consequence, although the output circuit can control the DC offset voltage, the circuit has a complex structure, which results in a consequent problem of increased manufacturing cost of an IC that incorporates the BTL amplifier.
It is, therefore, an object of the present invention to provide a balanced output circuit for generating both an inverted and a non-inverted output signals in response to an input signal such as an audio signal, the circuit having a simple circuit structure, yet capable of properly annihilating the DC offset voltage that otherwise exists between the two output signals. It is another object of the invention to provide an electronic apparatus such as a portable telephone equipped with an inventive balanced output circuit and a prestage circuit.